1. Field of the Invention
This invention relates to a scanning optical apparatus and an image forming apparatus using the same. The invention is particularly suitable for an image forming apparatus such as a laser beam printer (LBP) or a digital copying apparatus suited for highly minute printing in which a diffraction optical element is provided in at least one of a first optical system disposed on a light source means side relative to a light deflector and a second optical system disposed on a surface to be scanned side, whereby an aberration fluctuation (a change in focus) does not occur even if an environmental fluctuation (particularly a temperature change) occurs to the apparatus.
2. Related Background Art
Heretofore, in a scanning optical apparatus used in a laser beam printer or a digital copying apparatus or the like, a light beam light-modulated and emitted from light source means in conformity with an image signal has been periodically deflected by a light deflector comprising, for example, a rotatable polygon mirror, and has been converged into a spot-like shape on a photosensitive recording medium (photosensitive drum) by a scanning optical element (imaging element) having the f.theta. characteristic, and has optically scanned the surface thereof to thereby effect image recording.
A scanning optical apparatus of this kind in which a diffraction optical element is applied to the scanning optical system thereof is proposed, for example, in Japanese Laid-Open Patent Application No. 3-125111. In this publication, the optical system is comprised of a holographic f.theta. lens having power only in a main scanning direction, and a cylindrical lens having power in a sub-scanning direction. The correction of the f.theta. characteristic and surface inclination and the separation of first-order diffracted light and diffracted lights of other orders are effected.
Also, U.S. Pat. No. 5,486,694 discloses a scanning optical system having a surface inclination correcting effect in which a diffracting surface is provided on a toric lens.
However, the cylindrical lens in Japanese Laid-Open Patent Application No. 3-125111, when it is formed of a glass material, is more advantageous in cost than a conventional f.theta. lens comprising a combination of a plurality of lenses, but the cylindrical lens of a glass material is still expensive as compared with plastic lenses. On the other hand, if a cylindrical lens is formed of a plastic material to reduce the cost thereof, defocus in the sub-scanning direction is caused by an environmental fluctuation, particularly, a temperature change. To prevent this, the cylindrical lens can be brought close to a scanned surface, but if this done, the cylindrical lens will become large and as the result, the entire apparatus will become bulky, and this leads to the problem that the cost becomes high.
U.S. Pat. No. 5,486,694 suffers from the problem that no consideration is paid to the deterioration of aberrations caused by a change in the angle of diffraction due to the fluctuation of the wavelength of a laser beam.
On the other hand, in a scanning optical element (f.theta. lens) in a scanning optical system, the mainstream is to use a plastic lens in order to meet the requirements for lower costs and compactness.
FIG. 1 of the accompanying drawings is a schematic view of the essential portions of a scanning optical apparatus of this kind according to the prior art. In FIG. 1, a divergent light beam emitted from light source means 81 is made into a substantially parallel light beam by a collimator lens 82, and this light beam (the quantity of light) is limited by a stop 83 and enters a cylindrical lens 84 having a predetermined refractive power only in a sub-scanning direction. That part of the parallel light beam having entered the cylindrical lens 84 which is in a main scanning cross-section emerges intactly as a parallel light beam. Also, that part of the parallel light beam which is in a sub-scanning cross-section is converged and imaged substantially as a linear image on the deflecting surface (reflecting surface) 85a of a light deflector 85 comprising a rotatable polygon mirror. Here, the main scanning cross-section refers to a light beam cross-section formed with the lapse of time by a light beam deflected and reflected by the deflecting surface of the light deflector. Also, the sub-scanning cross-section refers to a cross-section containing the optical axis of an f.theta. lens and orthogonal to the main scanning cross-section.
The light beam deflected by the deflecting surface of the light deflector 85 is directed onto a photosensitive drum surface 87 as a surface to be scanned through a scanning optical element (f.theta. lens) 86 having the f.theta. characteristic, and the light deflector 85 is rotated in the direction of arrow A, thereby optically scanning the photosensitive drum surface 87 in the direction of arrow B. Thereby, image recording is effected on the photosensitive drum surface 87 which is a recording medium. Each of the collimator lens 82, the stop 83 and the cylindrical lens 84 constitutes an element of a first optical system L.sub.81, and the f.theta. lens 86 constitutes an element of a second optical system L.sub.82.
In the scanning optical apparatus of this kind, it is often the case that, as previously described, in order to meet requirements for the lower costs and compactness of the f.theta. lens, the f.theta. lens is comprised of only a plastic lens of an aspherical shape. The plastic material, however, has the nature that the refractive index thereof changes with the fluctuation of its environment of use (particularly, the fluctuation of temperature) and therefore, in a scanning optical apparatus using an f.theta. lens comprising the plastic lens, a change in focus is caused by the environmental fluctuation. Under the existing circumstances, the amount of focus fluctuation is set to such a degree that will not affect the actual image and therefore there arises no problem. When it is desired to step down the spot diameter of a condensed beam on the scanned surface for the purpose of more highly minute printing or when an attempt is made to bring an f.theta. lens close to the light deflector in an attempt to downsize the f.theta. lens for the purpose of lower cost and compactness, there is the problem that the above-mentioned focus fluctuation, particularly, focus fluctuation in the sub-scanning direction, cannot be allowed.